We have evaluated the efficacy of the antiestrogen tamoxifen in six men with painful idiopathic gynecomastia. Subjects were given either tamoxifen or placebo for 2 to 4 months and then were given the other agent for an identical period. Breast size was considered to have been reduced only if it had decreased by one or more Marshall-Tanner stages during the treatment period. Pain reduction with tamoxifen therapy was statistically significant for the group, occurring in five of six subjects during tamoxifen treatment and in only one of six during the placebo period. Size reduction with tamoxifen was only marginally significant for the entire group, but occurred in all three subjects who were initially in Marshall-Tanner stage III and in none of the three subjects who were initially in stage V. During tamoxifen treatment, there was a significant increase in the serum levels of luteinizing hormone and total estradiol and a marginally significant increment in the total testosterone level.